Why is nobody sitting in the dark blue seats?

Short answer

They are priority seats — dark blue, next to the doors, in every single carriage, kept for elderly, pregnant and disabled passengers. Nothing forbids sitting there; the expectation is that you stand up the moment someone needs it, and the norm is strong enough that a full train will still leave them empty.

Dark blue priority seats in a Taipei Metro carriage

What the blue seats are

Taipei Metro installs priority seats in every carriage, next to the doors, and colours them dark blue so they are obviously different from the rest. They exist for elderly, pregnant and disabled passengers.

The reason a crowded carriage still has empty ones is not a rule but a habit that holds: people would rather stand than risk sitting where someone else may need to be.

The ask may arrive as a sticker

Since September 2007 the operator has issued priority stickers at station information counters, for passengers who need a seat but do not visibly look as though they do. The campaign teaches everyone else to read the sticker and offer the seat.

So the request can be silent. If someone wearing one comes to stand near you, that is the ask, and the answer is to stand up without waiting to be spoken to.

The rest of the carriage has rules too

Give up your seat to anyone who needs it, and make space when a wheelchair comes aboard. Do not put luggage where it blocks other people, and take a backpack off your shoulders rather than swinging it into the person behind you.

Lower your voice on the phone. Move into the middle of the car rather than clustering at the doors, hold a handrail when the train is accelerating or braking, and do not lean against the doors or force through them once the closing buzzer sounds.